The preparation of cavities for acceptance of filling material is normally one of the very painful experiences encountered by a patient being subjected to the practice of dentistry. Attempts have previously been made to cool a tooth in which such a preparation is being undertaken by means of a dental assistant spraying water upon the operation, as well as many types of dental handpieces having water jets which operate to direct a stream of water at the area of the tooth where drilling is taking place. None of these systems actually direct cooling fluid such as water into the actual cavity where the cutting is taking place by the bur of the handpiece. Many modern handpieces also operate at very high speeds, as much as 400,000 rpm. Handpieces operating at such speeds require much less pressure than those operating at slower speeds, such as when they are driven by belts and the like.
It has been found by acurate scientific instruments that handpieces operating at such high speeds as mentioned above, involve problems with respect to generating heat in the tooth where the preparation is being formed. Laboratory measurements show a temperature at the bur-tooth interface of 450 degrees F. with a bur pressure of 32 grams at a bur speed of 170,000 rpm, after a running time of 30 seconds. The pulp section of a tooth contains nerves, blood vessels and lymph-carrying vessels, which are destroyed by high temperatures. Fillings placed over such damage can result in infection and abscesses. To alleviate this situation, cooling water jets are built into handpieces with their sprays directed at the cutting bur tip. However, laboratory tests show that little or no water contacts the bur tip per se because of turbulant air surrounding the same. It has been found that a minimum water flow rate of 16 cc/minute is necessary to form a stream with force enough to reach the bur tip area in a commercial 4-hole handpiece. This quantity of water floods the operating area, fogs the dental mirror, and reduces the view of the operating site. Contaminated water mist and sprays exit from the patient's mouth and settle on persons in the general area, creating a serious health hazard, especially when diseases such as AIDS and hepatitus may be present. It has been found that when the bur cuts below the outer surface of the tooth, cooling water entering that area is blocked by the projecting surfaces of the tooth and uncontrolled heating results.
Attempts have previously been made to direct water to the tip of a rotating bur by providing dental burs which have longitudinal passages extending entirely through the bur from one end to the other, but the problems mainly resulting from the use of such burs in dental handpieces reside in difficulties encountered when endeavoring to restrict cooling liquid, such as water, solely through the longitudinal opening in the bur, while in most instances, some of the cooling liquid enters the interior of the head in the vicinity of the turbine, under which circumstances, the lubricant normally required by the bearings of so-called air-rotor handpieces is flushed from the bearings or at least diluted by such water.
It is the principal object of the present invention to provide a dental handpiece which includes means to direct cooling liquid, such as water, through a bur which is provided with a longitudinal passage between opposite ends and discharge the water from the outer end of the bur directly to the interface between the bur and the tooth and improved means are included which are highly adequate to prevent the cooling liquid from reaching and/ or admixing with air turbines and the bearings therefor, details of which are set forth below.
To illustrate the present state of the art in which dental handpieces are provided with burs having axial passages between opposite ends of the bur, and means to direct cooling liquid thereto or therethrough, the following patents are cited as representative of the current state of the art on this subject:
______________________________________ U.S. Pat. No. 2,777,702 - Rodal Jan. 15, 1957 U.S. Pat. No. 2,799,934 - Kern July 23, 1957 U.S. Pat. No. 3,624,905 - Barsby Dec. 7, 1971 U.S. Pat. No. 3,762,052 - Melde (1) Oct. 2, 1973 U.S. Pat. No. 3,871,097 - Melde (2) Mar. 18, 1975 U.S. Pat. No. 4,021,920 - Kirschner May 10, 1977 U.S. Pat. No. 4,869,668 - Seney Sept. 26, 1989 ______________________________________